The present invention relates to nuclear reactors of the molten combustible salt type, and more specifically of the type having a primary integrated circuit. In such reactors the core and its reflector are located in a first vessel called the "reactor skirt" which is itself contained in its tower with pumps and primary exchangers in a second vessel called the main vessel.
These reactors use a liquid fuel heated to a high temperature of at least about 600.degree. C. by nuclear fission in the core, whereby said fuel is generally constituted by uranium or plutonium fluoride or a mixture of uranium fluoride and thorium dissolved in lithium fluorides and beryllium so that the mixture has a relatively low melting point, a suitable fluidity and a low vapour tension. When neutrons have to be thermalised the core of such reactors contains a suitable moderator mass such as graphite, whereby there are discharge channels for the combustible salt which then exchanges the calories obtained on passing through the core in a primary heat exchanger with another molten salt called the buffer salt, for example sodium fluoborate. In turn this buffer salt exchanges its calories in a secondary circuit comprising a steam generator, the steam being finally expanded in an electricity production plant.
In such integrated reactors the molten combustible salt is contained in a metal vessel called the main vessel whose walls are protected against chemical corrosion and high temperatures by carbonaceous materials. For example, this is the case with the nuclear reactor forming the object of French Patent Application EN 7442767 of 24.12.1974 entitled "Molten combustible salt nuclear reactor". In said reactor the main vessel is almost entirely filled with said carbonaceous materials and the only cavities provided in said carbonaceous material mass are those containing the reactor core, the heat exchangers, the circulating pumps for the molten salt and the different galleries ensuring the hydraulic connection between the components specified hereinbefore.
In theory, this design of the integrated primary circuit in a main metal vessel permits the insulation of said confining structure relative to the molten combustible salt contained therein.
Unfortunately, experience has shown that although interesting in theory these constructions give rise in practice to a certain number of shortcomings due in particular to the great difference existing between the thermal expansion coefficients of the vessel material on the one hand and the carbonaceous filling material on the other. As a result, it is necessary to provide a better protection of the main vessel relative to the molten salts. A device for protecting a main vessel of this type forms the object of French Patent Application EN 7517939 filed on June 9, 1975 entitled "Process and apparatus for protecting the vessel of a molten salt nuclear reactor".
A per se known device of this type will be described in greater detail with reference to FIG. 1.
FIG. 1 shows in the form of an axial half-section a molten salt reactor having an integrated primary circuit designated by the general reference numeral 1. In a protective concrete enclosure 2 said reactor comprises a main vessel 3 containing the reactor core 4 itself located in the reactor skirt 5 and the exchangers and circulating pumps for the molten salt, whereby the location of the single exchanger 6 is clearly visible in FIG. 1. The inner wall of said main vessel 3 as well as its internal volume not occupied by the reactor elements are lined with a per se known carbonaceous filling material designated by 7 in FIG. 1.
In this construction a second outer vessel 8 is placed around the main vessel 3, whereby the space between vessels 3 and 8 is filled with a climatisation or air-conditioning fluid 9. The temperature of this fluid is controlled, for example by at least one submerged circulating fluid exchanger 10, the water entering at 11 and leaving in the form of steam at 12.
Firstly, the main vessel is heated to a temperature of, for example, 400.degree. C. by acting on the air-conditioning fluid 9 and exchangers 10 leading to the formation of an empty space 13 as a result of the differential expansion occurring between carbon 7 and the steel of vessel 3. Vessel 3 is then first filled with an auxiliary salt which contains no fissile or fertile material and whose melting point is below the temperature to which vessel 3 has been heated, i.e. 400.degree. C. in the case described herein. This salt can be of different types and advantageously it is constituted by the eutectic of lithium fluorides and beryllium whose melting point is 350.degree. C. It is absolutely necessary for it to be chemically compatible with the actual combustible salt. This auxiliary salt which is neutral from a nuclear standpoint fills the said empty space 13 and the interstices located in the carbonaceous lining mass 7.
In a second phase which follows the first the temperature of the main vessel 3 is lowered to below the melting point of the auxiliary salt used by means of the air-conditioning fluid 9 and the exchangers 10 which causes the agglomeration or solidification of that part thereof which has filled space 13 created between the main vessel 3 and the carbonaceous filling material mass 7 and to a certain depth the interstices emerging at the periphery of the carbonaceous mass 7. In the present example where the eutectic of lithium fluorides and beryllium has a melting point of 350.degree. C. the air-conditioning temperature of fluid 9 is, for example, lowered to 300.degree. C.
When all the auxiliary salt in space 13 has solidified and this solidification has also penetrated a certain depth into the interstices emerging in said area 13, the reactor is loaded with the final combustible salt.
At the end of these operations vessel 3 is definitively maintained at 300.degree. C. and the reactor is ready for operation. The crust of auxiliary salt in the space 13 between vessel 3 and the carbonaceous filling mass 7 substantially has no contact with the combustible salt. Thus, there is no need to fear a nuclear reaction in said crust of neutral salt which is thus maintained at an essentially constant temperature and can effectively fulfil its function of providing corrosion protection for the main vessel 3.
The present invention applies to the molten salt reactor of the type described with reference to FIG. 1 and the invention in fact aims at improving the heat exchangers of reactors of this type.
In such reactors the primary heat exchange system has hitherto been constituted by an integrated system in the main vessel and has comprised the arrangement in alternate manner of an exchanger and a pump in the carbonaceous filling material, whereby said components are distributed over the entire periphery of the structure. In the known constructions, the pumps circulate the molten salt in the reactor skirt from bottom to top and the hot salt descends in countercurrent flow by forced circulation in the adjacent exchangers which it thus traverses from top to bottom. A transverse hydraulic connection is then necessary between the base of each primary exchanger and the adjacent pump shafts in order to ensure the return of the cold salt to the reactor core resulting in significant expansion problems in operation.